Unable to get X8 to fly, tried everything

[joelmeijering]

Hi all, I've been working on an octocopter for about nine months now, and I haven't been able to get it to take off at all. It's my first build.

I've exhaused all my options and I have no idea how to fix it.

These are my test results:

Test rig A: Constrained on all axis, with a few centimeters wiggle room
Result: With lowest throttle: starts to increase throttle automatically, starts shaking more and more aggresively. Had to abort test after 5 to 10 seconds depending on PID settings.

Test rig B Only constrained on roll axis, is able to flip 360 deg.
Result: With lowest throttle: flips immediatly. Overcompensates and flips aggressively, had to abort after 3 sec (a full flip was made already)

I've tried everything I could think of, including:
- Tried various PID's, didn't have any effect
- Used various test rigs
- Verified all motors and props are wired properly
- Enabled deadband to ensure sticks are centered when taking off
- Rebuilt frame to put motors futher away for stability
- Tried different props, batteries and esc's
- Different FC, same issue

Specs are:

Class: ~70cm Coax Quadcopter X
Weight: ~3.5KG
Frame: H-shaped ABS plastic
Battery: 5000mAh 4S Turingy
Propeller: 8x 12x6 APC clone
ESC: 8x Chinese simonk 30A/3-4S
Motor: 8x SunnySky V2814 800KV
Flight controller: Crius MWC MultiWii SE V2.5, Loaded with MW 2.3
RX/TX: Spektrum DX6i, AR6210

Does anyone have a suggestion?

Thanks,

Joel

[nebbian]

Firstly, set I and D parameters to 0.
Disconnect power from all bottom motors.
Take all props off.
Ensure that you are in acro mode (no self levelling).
Connect a battery (not a power supply).
Slowly throttle up so that the motors are just above idle, and verify that moving the aileron right/left makes the correct motors speed up. Do the same for pitch.
Now grab the frame and move it quickly in the roll axis, ensure that the correct motors are speeding up to counteract the roll. Do the same for pitch.

If all is working properly, put some props on the top 4 motors, constrain with your test stand (allowed to roll) and repeat with a slow throttle up and see if it is stable.

Once you've got all that sorted out, power down, and repeat the process with only the bottom motors connected.

Then, and only then, connect all 8 props and try again.

From the image it looks like two props are on upside down (the bottom far right and far left on your picture). Hard to tell though.

Good luck.

[joelmeijering]

Thanks for your reply, I followed you instructions and I might be on to something! But still have no clue whats going wrong.

So first I did the proplless checks, all was OK:
With props removed and in acro mode:
- the motors properly counteract to roll movement
- adjusting roll stick correctly spins up the right motors

Then I added the 4 top props and enabled Acro mode. It now does not flip and responds to roll input! I had to put I and P higher than 0 though, otherwise I ignores my roll stick input.

So somehow Horizon mode seems to cause problems, while Acro seems to work fine. But I need horizon mode. What could be causing this problem?

[nebbian]

Thanks for your reply, I followed you instructions and I might be on to something! But still have no clue whats going wrong.

So first I did the proplless checks, all was OK:
With props removed and in acro mode:
- the motors properly counteract to roll movement
- adjusting roll stick correctly spins up the right motors

Then I added the 4 top props and enabled Acro mode. It now does not flip and responds to roll input! I had to put I and P higher than 0 though, otherwise I ignores my roll stick input.

So somehow Horizon mode seems to cause problems, while Acro seems to work fine. But I need horizon mode. What could be causing this problem?

You do need P values, just not I or D initially. Setting them to zero helps to eliminate some tuning issues. Once you've figured stuff out you can put them back.

Speaking of which, you've managed to faultfind the problem, well done! You need to tune the level parameters.

If using the default pid controller (if you don't know what I'm talking about, then you are using it), then play around with the level settings. Specifically, lower I, and raise D. P might need to go up or down, hard to tell from your description, probably up. Try one thing at a time, and go the other way if it makes things worse.

[joelmeijering]

Thanks, today I rearanged components and initilized the FC while being exacly level. After these thanges, at default PID values and Horizon mode, it stabilizes on the roll axis! Finally I can start tuning the PIDs.

Thanks a lot!